The present invention relates generally to circuits which provide a hysteresis function, and more specifically relates to circuits used in anti-lock braking systems(ABS).
ABS used in conjunction with air-braked truck trailers are in common use. An objective of an ABS is to allow the wheels of a vehicle to continue to rotate during braking, including heavy braking. Keeping the wheels moving during braking generally provides more efficient braking. As shown in FIG. 1, an ABS generally consists of wheel speed sensors 10, an Interface circuit 12, an Electronic Control Module(ECM) circuit 12, an Electronic Control Module(ECM) 14, a Pneumatic Control Module (PCM) 16, and brake mechanisms 18 associated with the wheels of the vehicle. The ECM 14 is sometimes referred to as the Electronic Control Unit (ECU). The PCM 16 is sometimes referred to as the modulator and, in some anti-lock braking systems, the PCM is integrated into another component such as a relay valve.
An ABS for air brakes generally works as follows. Each wheel speed sensor 10 measures the speed of a wheel and forwards this information, in the format of an electrical signal, through the interface circuit 12. The interface circuit 12 transforms the electrical signal produced by the wheel speed sensor 10 into a square wave. The square wave is then used by the ECM 14 to calculate wheel speed information. The wheel speed information is then transmitted to the PCM 16 which sends instructions to the braking mechanisms 18 which modify the air pressure to control the braking level. This braking process is well known and is described in numerous patents and in pending, U.S. patent application Ser. No. 09/306,921, which is hereby incorporated herein in its entirety by reference.
One type of wheel speed sensor which is commonly used in ABS systems is a variable reluctance (VR) sensor. A typical VR sensor 20 is,shown in FIG. 2, and consists of a permanent magnet 22, a magnetically soft pole piece 24 and a coil of wire 26 wound around the pole piece 24. A magnetic field extends from the permanent magnet 22, through the pole piece 24 and coil 26 out into the air space proximate the face 31 of the VR sensor 20. The return path of the magnetic field is from the air space proximate the face 31 to the opposite end of the permanent magnet 22.
Each end of the coil 26 is attached to a cable 28 which extends through the sensor housing 30 opposite the face 31 of the VR sensor 20. The electrical signal produced by the VR sensor 20 flows through the cable 28. Another type of sensor commonly used in ABS""s is a Hall sensor, which construction is generally known in the art.
As shown in FIG. 3, when a VR sensor 20 is employed in an ABS, the VR sensor 20 is typically associated with a toothed wheel 32 (sometimes referred to as an exciter ring or a tone wheel). Usually in the truck and trailer industry, the toothed wheel is attached directly to the back of the wheel hub 34 and rotates with the road wheel (not shown). The VR 20 is mounted so that the face 31 of the VR sensor is proximate the toothed wheel 32 and perpendicular to the axle 38.
The toothed wheel 32 includes a row of teeth 40 around the perimeter of the toothed wheel 32. A gap 42 is located on either side of each tooth 40 along the perimeter of the toothed wheel 32. As the road wheel rotates, the teeth 40 of the toothed wheel 32 pass the face 31 of the VR sensor 20. Because the toothed wheel 32 is made of a ferrous material, as each tooth 40 approaches the face 31 of the VR sensor 20, the magnetic field increases. As each tooth.40 becomes further away from the face 31 of the VR sensor 20, the magnetic field decreases. The magnetic field, or flux, is greatest when the tooth 40 is in front of the VR sensor 20. Conversely, when a gap 42 is in front of the VR sensor 20, the flux is least. Thus, as the teeth 40 pass the face 31 of the VR sensor 20, the flux increases and decreases.
Through basic electromagnetic theory, this changing flux induces an AC voltage signal across the coil 26. The induced voltage is ideally a sinusoidal signal. The frequency of the signal is directly proportional to the number of teeth 40 which pass the face 31 of the VR sensor 20 in a given period of time. The amplitude of this voltage signal is proportional to the speed of the teeth 40 passing the face 31 of the VR sensor 20. When the road wheel is turning at high speeds, the AC signal has a high amplitude. When the road wheel is turning at low speeds, the AC signal has a low amplitude. As the wheel speed becomes very slow, the signal becomes generally unreliable. Typically, designers of ABS""s assume that when the wheel speed is less than 2 mph, the signal, received from each wheel sensor is unreliable. The speed at which the signal becomes unreliable, however, is dependent upon many factors of the overall system design, including, for example, the sensitivity of the circuit, the VR sensor, the toothed wheel, and the gap maintained between the face of the VR sensor and the toothed wheel.
Because the signal received by the ECM will be used to generate wheel speed information, it is critical that this signal be as accurate as possible. However, certain factors create imperfections in the signal. These imperfections can result in the ECM 14 incorrectly calculating the wheel speed.
One factor which adversely affects the sinusoidal signal produced by the VR sensor is mechanically induced noise. At times, even though the vehicle is moving, the road wheel is not rotating. The road wheel therefore rubs on the road surface producing a noise sometimes referred to as tire scrub. This noise may additionally be amplified by suspension resonances. Because sensors, such as VR sensors, are generally prone to mechanically induced noise, the sensor will produce significant AC voltage even though the wheel is stationary. Therefore the sensor will send an output signal which indicates that the road wheel is rotating when, in fact, it is stationary. This situation is further complicated by the fact that the mechanical noise tends to be at a relatively high frequency. Thus the high frequency noise causes a great rate of change of flux. Because a VR sensor, for example, responds to the rate of change of flux, the resulting signal will have significant frequency imperfections.
Other factors such as electrical interference effects from onboard or off board radios, radars and other radio frequency interference affect the overall sensing scheme. Depending on the severity, these effects may combine and prevent the ECM 14 from operating correctly and imperfections in control performance may result. When greater degradation of the signal occurs, the ECM 14 determines that the signal is unworkable and the ABS system shuts down.
Another factor which leads to imperfections in the signal produced by the VR sensor is a varying gap between the face 31 of the VR sensor 20 and the toothed wheel 32. As shown in FIG. 3, in the heavy truck and trailer industry, a VR sensor 20 is oriented along the axle of the vehicle resulting in the face of the VR sensor 20 being perpendicular to the wheel hub 34 on which the toothed wheel 32 is mounted. Axial slack causes the gap between the VR sensor 20 and the toothed wheel 32 to vary. When the gap between the VR sensor 20 and the toothed wheel 32 is large, the amplitude of the AC voltage signal is low. When the gap is small, the amplitude of the AC voltage signal is larger. This results in amplitude modulation of the sinusoidal waveform due to variation of the sensor gap.
An additional effect of the varying sensor gap is best understood by considering the concept of a xe2x80x9ctoothlessxe2x80x9d tone wheel. Initially, one might conclude that in the case of a toothless wheel, as the wheel rotates no signal would be generated, as there is no variation in the magnetic flux. However, it is clear that as the tone wheel moves axially, the sensor gap varies. Consequently, the magnetic flux also varies and a voltage output from the sensor results. When an actual (xe2x80x9ctoothedxe2x80x9d) tone wheel is used, this voltage output is reflected as shifts in the average voltage level of the speed signal. These shifts are typically of a lower frequency than that of the speed signal itself.
The movement of the hub 34 therefore causes imperfections to the sinusoidal waveform. The varying gap therefore results in both amplitude modulation and additional lower frequency components.
It is desirable to eliminate these noise components from the signal transmitted to the ECM 14. However, it is also desirable to track the wheel speed to as low a speed as possible, in order to optimize the performance of the ABS system. These two requirements are in conflict.
Typically, ABS""s accomplish this compromise by implementing basic hysteresis functionality in the interface circuit. The basic hysteresis concept proposes that if an effect happens as a result of an increasing stimulus then, for the effect to be reversed, the stimulus must be reduced below the level which caused the effect to occur in the first place.
This hysteresis concept can be implemented in an ABS system to eliminate some of the noise from the signal produced by the wheel sensors 10 (See FIG. 1). The signal produced by each sensor is generally a sinusoidal signal and carries wheel speed information along with a noise component. This signal, generated by the wheel sensors, is transferred to the interface circuit 12. FIG. 4 represents an interface circuit 12 commonly used to implement hysteresis functionality. The signal generated by the wheel speed sensors is received by the interface circuit at the input nodes 44, 46. The interface circuit 12 typically uses a comparator 62 which switches between two stable states (high and low) as the input signal oscillates. As the comparator 62 switches between its two stable states, a square wave is generated at the comparator""s output 102. It is this square wave, from the output of the comparator 102, which is then used by the ECM 14 (see FIG. 1) to calculate the wheel speed.
Basic hysteresis functions such that, the signal received by the comparator 62 from the input nodes 44, 46 must be of sufficient amplitude in order to switch the output 102 of the interface circuit 12 from one stable state to the other. The noise portion of the signal is generally not of sufficient amplitude to cause the comparator""s output 102 to switch. Thus, if the comparator""s output is in the high state, the noise portion of the signal is not generally of sufficient amplitude to cause the comparator 62 to switch to its low state and no change occurs at the output. Because the noise portion of the signal generated by the VR sensor does not affect the comparator""s output 102, this basic hysteresis circuit functions so as to essentially eliminate the noise portion of the signal produced by the VR sensor from the output signal.
Although the circuit shown in FIG. 4 is functional, typically additional protection circuitry would be added for long term reliability in the automotive environment. This implementation is well known to those familiar with the art.
Although implementation of this basic hysteresis function eliminates noise from the interface circuit output signal 102, one problem which is encountered through its use, is the loss of low level input signals, which may not be due to noise. Because the amplitude of the signal produced by the VR sensor is proportional to the speed at which the road wheels are rotating, low level signals produced by the VR sensors are often simply an indication that the wheel is rotating slowly. Although this low amplitude signal will be transmitted to the interface circuit 12 when the hysteresis function is implemented, the signal generated by the VR sensor will not have sufficient amplitude to cause the output 102 of the interface circuit 12 to switch states. Thus, although the road wheel is rotating, albeit slowly, the signal produced at the output 102 of the interface circuit 12 indicates that the wheel is not rotating.
Likewise, when the road wheel is rotating, and axial slack causes a gap between the toothed wheel 32 (see FIG. 3) and the VR sensor 20, the signal produced by the VR sensor 20 may not have sufficient amplitude to cause the output 102 of the interface circuit to switch states. Although the wheel is rotating, the signal produced by the interface circuit 12a indicates that the road wheel is not rotating.
Unfortunately, this inaccurate information indicating that the wheel is not rotating is then passed to the ECM 14. The ECM 14 in turn relays the information to the PCM 16 which may make improper modifications to the air pressure level at the brake chamber 18. This is not particularly a problem if the vehicle is moving very slowly. At such a low speed, whether the ABS functions or not is largely irrelevant. However when the vehicle itself is moving at a higher speed it is important that the wheel speed can be tracked to as low a speed as possible so that optimum ABS control can be maintained. It is highly desirable that the ECM 14 continue to receive wheel speed information from the sensors 10 so that proper braking instructions can be given.
Thus, it would be desirable if there was a high hysteresis level when no signal is present but a lower level once the signal was detected. This would be particularly helpful in rejecting the tire scrub induced noise discussed above but would also minimize the problem of losing the signal at low speeds. At very low speeds no signal would be present, but once the threshold speed was reached than the hysteresis would be deceased so that some reduction in signal amplitude would be tolerated. In effect, there would be hysteresis applied to the hysteresis level.
A general object of an embodiment of the present invention is to provide a circuit configured so as to apply variable hysteresis to an incoming signal.
An object of an embodiment of the present invention is to provide a circuit capable of generating a more accurate output signal.
Another object of an embodiment of the present invention is to provide a circuit configured to apply variable hysteresis to an incoming signal so as to track lower vehicle wheel speeds.
A further object of an embodiment of the present invention is to provide accurate wheel speed information to an ECM.
Another object of an embodiment of the present invention is to provide information to an ECM which will result in more efficient and effective braking of a vehicle.
Yet another object of an embodiment of the present invention is to eliminate noise from the signal produced by a wheel speed sensor.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the invention provides a circuit which is configured to apply variable hysteresis to an input signal, such as to a signal which is generated by a wheel speed sensor in an ABS. The level of hysteresis which is applied to the input signal is dependent upon the frequency of the signal generated by the wheel speed sensor. Preferably, the circuit includes a comparator configured to provide positive feed back.